The present invention relates to the field of telecommunications and more particularly to a method for controlling the transmitter part of a radio transceiver in order to correct power drift or amplifiers non-linearity.
In the description below, the term xe2x80x9cradio transceiverxe2x80x9d refers to a radio terminal as well as to a base station of a radio communication network. The following invention is applicable as well in fixed as in mobile radio communication networks.
In a radio communication network, resources are allocated to a radio transceiver on a communication channel to transmit/receive information signals to/from the radio communication network. Resources are preferably allocated according to time-sharing principles. However, code-sharing, frequency-sharing, or a combination of them with time-sharing are also compatible with the present invention.
Radio communication networks requires usually that their radio transceivers fulfil criteria regarding transmitted power. Such constraints are set to avoid interference between different radio transceivers and to optimize the load on the radio interface. For this purpose, a possible drift of the transmitted power at the radio transceiver has to be avoided. Non linearity of amplifiers are also impairments of the transmitter part and should be compensated to limit for example inter-modulation effects. Some modulation types, for example the modulation used in EDGE system (Enhanced Data rate for GSM Evolution) are more sensitive to non linearity in the transmitter part of the radio transceiver resulting in a quality degradation. As a consequence, a dynamic control of the transmitter part of a radio transceiver may be necessary to guaranty good performance in the whole radio communication network.
EP-0,655,179 describes a method for testing a transceiver of a time divisional radio communication system. This method consists in assigning at least two free time slots for use selected in such a manner that when the transmitter transmits on the first selected time slot, the receiver simultaneously receives the second selected time slot. A predefined test signal is supplied to the input of the transmitter using the first selected time slot. The signal is first adapted to the reception frequency of the receiver and received in the second selected time slot at the receiver. The transceiver compares the signal obtained at the output of the receiver with the test signal supplied to the transmitter.
A drawback of this method is the use of a predefined test signal which has to be sent in a time slot available as well at the transmitter as at the receiver.
A particular object of the present invention is to obviate the problem described above.
Another object of the present invention is to provide a method for quantifying the transmitter part impairments.
This object, and others that shall appear below, are achieved by a method for controlling the transmitter part of a radio transceiver, the transmitter part comprising an IQ modulator, the radio receiver further comprising a receiver part comprising an IQ demodulator, the radio transceiver being dedicated to transmit modulated information signal over an air interface, the method being characterized by comprising the steps of:
storing an IQ modulation vector obtained at IQ modulator and associated to a portion of the information signal;
looping the portion of information signal to the receiver part, the information signal having crossed a simulated radio interface between the transmitter part and the receiver part;
estimating an error vector by comparing the IQ modulation vector to an IQ demodulation vector obtained by demodulation of the portion of information signal at IQ demodulator; and
modifying transmitter part parameters according to the error vector.
An advantage of the present invention is to use a portion of the transmitted information signal itself to detect and compensate impairments in the transmitter part of the radio transceiver. As a consequence, there is no more need for a predefined separate test signal.
Another advantage of this invention is that the transmitter part parameters can be easily controlled by adding the latest error vector to IQ modulation vectors associated to subsequent portions of the information signal.
The present invention also concerns a radio transceiver implementing the above described method. The radio transceiver comprises a transmitter part with an IQ modulator, and a receiver part with a IQ demodulator, the radio transceiver being dedicated to transmit an information signal modulated by said IQ modulator through an air interface, the radio receiver being characterized in that it further comprises:
a derivation to loop a portion of an information signal dedicated to be transmitted over the air interface from the transmitter part to the receiver part over a simulated radio interface;
an error vector estimator comparing an IQ modulation vector associated to said portion of information signal to an IQ demodulation vector obtained by demodulation of said looped portion of information signal at the IQ demodulator; and
a transmitter controller modifying parameters of the transmitter according to the error vector.
This invention is based on a priority application EP 00 44 01 32 which is hereby incorporated by reference.